Basis of Presentation and Significant Accounting Policies - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ 550,080	$ 481,532	$ 429,512	$ 404,112	$ 354,533	$ 294,149	$ 242,874	$ 235,701
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	6,389			8,769
Net unrealized gain on derivative financial instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	104			64
Unrecognized prior service cost on benefit obligations
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(14,707)			(16,614)
Net unrealized gain (loss) on available-for-sale investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(194)			333
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ (8,408)	$ (17,519)	$ (18,645)	$ (7,448)	$ (7,322)	$ (4,932)	$ (9,316)	$ (10,289)